The M locus as described in the mouse is distinguished by the fact that, aside from the H-2 complex, it is the only other genetic locus which is capable of stimulating strong primary mixed lymphocyte responses as measured by proliferation and the secretion of T cell growth factor, or Interleukin-2 (IL-2). That these responses involve the activation of an extraordinarily high frequency of T lymphocyte precursors suggests the M locus is important biologically. Central to this is the question of whether the M locus represents a duplication of one or more H-2 genes translocated to another chromosome in the mouse or is, in fact, a unique genetic complex in its own right which exists in all species. As a corollary, is the manner in which T cells recognize M locus antigens different from the way(s) in which other antigens are recognized, and related to the strong activating properties of the M locus? The use of mixed cell populations to describe the M locus, however, has provided only an imprecise characterization in biological terms. Consequently, much of what is known about the M locus is only suggestive. In this regard, techniques of cloning or cell fusion can now provide homogeneous lines of responding and stimulating cells for analysis. Using such reagents and the secretion of IL-2 as a functional assay for antigen recognition, we propose in these studies to define the M locus in terms of 1) the requirements for T cell recognition, 2) the number of loci controlling expression, 3) antigenic structure, 4) the expression of similar structures in other species, and 5) the T cell structures associated with recognition of M locus antigens. The long-term objectives of these studies are to facilitate an understanding of why M locus antigens activate such a high frequency of T cells, their role in immune responses, and the identification of equivalent loci in other species.